Serum beta-N-acetylhexosaminidase levels in 49 patients with inflammatory bowel disease (IBD; 23 patients with ulcerative colitis, 10 with Crohn's disease, and 16 with ileostomy after total proctocolectomy) as well as in healthy normal controls were determined. They were found to be significantly elevated in 91% of all patients, disregarding the state of activity of the disease. It seems most likely that activated macrophages are the source of this lysosomal hydrolase in the serum of IBD patients since other theoretical possibilities, like damaged hepatocytes or elevated serum bile acid levels, are not relevant for these patients.